The present invention relates generally to an optical probe assembly, particularly an external optical probe assembly for use in determining a physical distance in a camera between two reference surfaces.
To capture an image of an object using a camera, an image beam is directed through a lens and onto an image-forming medium disposed at an image plane. In conventional photography, photographic film is disposed at the image plane and the image of the object is captured on photographic film. In electronic photography, an imager or image sensor (e.g., a Charge Coupled Device (CCD), or Complementary Metal Oxide Semiconductor (CMOS)), is disposed at the image plane, and the image of the object is captured electronically. For both conventional and electronic photography, it is necessary to properly position the image plane and the lens within the camera, relative to each other, to provide a focused image.
Commonly assigned U.S. Pat. No. 5,757,485, entitled, xe2x80x9cDIGITAL CAMERA IMAGE SENSOR POSITIONING METHOD INCLUDING A NON-COHERENT INTERFEROMETER,xe2x80x9d by Michael A. Marcus, et al., issued on May 26, 1998; and U.S. Pat. No. 5,757,486, entitled, xe2x80x9cDIGITAL CAMERA IMAGE SENSOR POSITIONING APPARATUS INCLUDING A NON-COHERENT LIGHT INTERFEROMETER,xe2x80x9d by Michael A. Marcus, et al., issued on May 26, 1998, relate to a digital camera image sensor positioning apparatus and method which includes a non-coherent light interferometer. The apparatus and method include an optical probe assembly that is removable and that can be mounted to a digital camera. Before mounting the optical probe assembly into the camera, the camera""s shutter must be first opened by actuating the camera""s electronics. A low coherence light interferometer is in communication with the optical probe assembly to determine a depth from a reference surface to the image surface and optical probe assembly.
The low coherence light interferometer can aid in measuring the exacting manufacturing specifications required for properly positioning image sensors like CCDs and CMOS image sensors. The steps in a CCD based imager manufacturing process are as follows. For a conventional CCD manufacturing process, multiple CCD imager arrays are processed together on a single silicon wafer. Imager dies, composed of a single CCD imager array, are diced from the wafer, positioned, and glued into specially designed packages. A flat transparent plate called the imager cover glass is then glued into the specially designed package at a location that is offset from the imager die to hermetically seal the specially designed package. This hermetically sealed package containing the imager die is then mounted into a camera mounting plate that includes a reference plane to facilitate proper mounting of the image sensor into the camera. The camera itself will include a camera reference plane that receives the camera mounting plate from the imager package. In a film camera, film rails usually define the camera reference plane. Optionally, the package can include the camera mounting plate and reference plane, which obviates mounting the hermetically sealed package into a camera mounting plate.
In order to ensure that the CCD is positioned properly in the camera, the exact location of the CCD needs to be determined. The CCD location can be determined relative to a reference surface or reference plane.
A Coordinate Measuring Machine (CMM) is an example of an apparatus employed to determine the location of an object relative to a reference plane. Conventionally, the object is retained in a suitable holder on an optical bench. In one method to determine the location of an object, three points on a reference plane, approximately 120 degrees apart, are measured to define the reference plane; the coordinates of the three points are tracked in the x, y and z directions. A point on the object is then measured relative to the reference plane, and the distance from the reference plane is calculated. Conventional CMMs have contact probes for intimately contacting each of the points defining the reference plane and the object.
Another technology known as low-coherence light interferometry has also been used to measure physical properties of an object. U.S. Pat. No. 5,659,392, entitled xe2x80x9cASSOCIATED DUAL INTERFEROMETRIC MEASUREMENT APPARATUS FOR DETERMINING A PHYSICAL PROPERTY OF AN OBJECT,xe2x80x9d by Michael A. Marcus, et al., issued Aug. 19, 1997, and U.S. Pat. No. 5,596,409, entitled, xe2x80x9cASSOCIATED DUAL INTERFEROMETRIC MEASUREMENT METHOD FOR DETERMINING A PHYSICAL PROPERTY OF AN OBJECT,xe2x80x9d by Michael A. Marcus, et al., issued Jan. 21, 1997, disclose an associated dual interferometric apparatus and method for measuring physical properties of an object, such as thickness, group index of refraction, and distance to a surface. U.S. Pat. No. 5,757,485 and U.S. Pat. No. 5,757,486, disclose a digital camera image sensor positioning apparatus and method which includes a low-coherence light interferometer. The apparatus and method include a removable optical probe assembly mounted to a digital camera. The low-coherence light interferometer is in communication with the optical probe assembly to determine a depth of an image sensor residing within a digital camera, relative to a reference surface. U.S. Pat. No. 6,075,601 describes an optical probe calibration apparatus used for calibrating the optical probes used in U.S. Pat. Nos. 5,757,485 and 5,757,486 referenced above. These three aforementioned U.S. patents require that the optical probe be mounted in the camera body in order to determine the location of the imager sensor with respect to the camera reference surface.
Therefore, a skilled operator is required to install the image sensor in the camera and subsequently assemble the camera, before finding out if the image sensor is properly focused. Several steps are required, including securing the image sensor with 3 or 4 screws onto the camera mounting plane, and inserting a measurement optical probe into the camera body and locking the probe into the lens flange mounting ring before a measurement can be initiated. Before mounting the measurement optical probe into the camera body, the camera electronics has to be turned on and the electronic shutter opened. Full camera assembly and substantial skilled operator intervention are required before an assessment of image sensor focus can be made. If the image sensor is out of focus, the camera is disassembled and the image sensor is replaced. In order to calibrate the measurement optical probe, an external calibration fixture is also required.
While internal apparatus and methods may have achieved a certain level of success, the internal apparatus is not readily transportable nor simple to use. Further, the methods are time consuming and quite often are dependent on the skill of the operator.
Accordingly, a need continues to exist for a simplified, user-friendly apparatus and method for determining the position of an image sensor in a digital camera. Furthermore, there is a need to properly predict the position of an image sensor before permanently physically mounting the image sensor inside the digital camera. The apparatus needs to be robust, transportable, and simple to use. The method must be fast, provide objective results independent of the operator, and provide accurate and consistent results.
In particular, for SLR cameras configured for use with 35 mm photographic film or APS format film, a need continues to exist for rapidly measuring a distance between a lens mounting member of the camera and film rails disposed at the image plane. Such a measurement assists in the determination of whether the SLR camera is within manufacturing specifications.
The aforementioned need is met according to the present invention by providing an interferometric-based measurement system for externally measuring a distance between a pair of reference surfaces on an object, that includes: a low coherence light interferometer; an object mounting apparatus including an optical probe having an optical probe chuck; an optical fiber cable for coupling light from the interferometer to the optical probe chuck; and a computer for processing data collected by the interferometer, wherein the data is used to determine the distance between the pair of reference surfaces on the object.
Another embodiment of the present invention provides an interferometric-based external measurement system for determining whether a distance between a measurement camera""s lens flange mounting ring and a camera""s second reference surface meets predetermined dimensional specifications, that includes: a low coherence light interferometer; a camera mounting apparatus, including an optical probe having an optical probe chuck; an optical fiber cable for coupling light from the interferometer to the optical probe chuck; and a computer for processing data collected by the interferometer, wherein the data is used to determine whether the predetermined dimensional specifications are met.
Yet another embodiment of the present invention provides a method for externally calibrating an apparatus that measures a physical distance in a camera between a first reference surface on the camera and a second reference surface (LFu) on the camera, the first reference surface being substantially parallel to the second reference surface including the steps of: a)mounting the first reference surface of a reference camera with a known distance LFr onto a measurement apparatus that includes an optical probe with an optical probe reference surface R, and an adjustable height measurement flat; b) clamping to mate the adjustable height measurement flat to the second reference surface of the camera (F);and c) determining a distance RBr with a low-coherence light interferometer, where RBr is defined as the distance from the optical probe reference surface R to a second planar parallel surface of the adjustable height measurement flat (B).
The present invention provides a method for determining a physical distance in a camera between two reference surfaces. The method is robust, transportable, simple to use, and can be readily used with both a digital camera and a film camera.